Five of the presently known six selenoenzymes are produced by bacteria and four of these are under investigation in this laboratory. These are selenoprotein A of glycine reductase, formate dehydrogenase of Methanococcus vannielii, nicotinic acid hydroxylase of Clostridium barkeri and thiolase of Clostridium kluyveri. In addition studies are underway on the identification and mode of formation of seleno-tRNAs (modified with Se in the polynucleotide portion of the molecule) which were first discovered in C. sticklandii. The role of selenium in the various selenoenzymes and the mechanism of incorporation of selenocysteine in selenoprotein A and in formate dehydrogenase are the subjects of our ongoing studies.